1. Field of the Invention
The present invention relates to an ideal display system, used in a display system where a host that outputs a digital video signal, or a host that outputs an analog video signal are connected to a display apparatus via respective specialized cables, and to a display apparatus and to a cable for connecting the display apparatus to a computer.
2. Description of the Related Art
Heretofore there is known a display system where, a host such as a computer graphics card that outputs RGB (red green blue) analog signals as video signals, and a similar host that outputs digital TMDS (Transition Minimized Differential Signaling) signals as video signals, are selectively connected via their respective specialized cables to a display apparatus, so as to display the video signals from each of the hosts.
Operating systems of current computers serving as the aforementioned hosts further comprise a plug and play function such that, when the display apparatus is connected to a host, that host reads the specification information of the display apparatus and selects the appropriate driver software corresponding to that specification information, and automatically carries out setting inside the host so as to perform the appropriate display.
The specification information that a plug and play supported display apparatus gives to the host side is known as EDID (Extended Display Identification Data). This includes information such as the resolution, synchronization signal frequency and serial number of the display apparatus, and is also data for the interface types with which the display apparatus is compatible, that is data which differs depending on whether the video signal is the aforementioned analog RGB (red green blue) signal or the digital TMDS signal. This EDID is transmitted to the host via an SCL clock line and an SDA data line called a DDC (Display Data Channel) communication line, inside the connection cable.
The aforementioned interface type has mainly been an analog interface type employing a D-sub connector (15-pin D-shell Display Connector) that handles analog signal. However DVI, advocated by the American VESA (Video Electronics Standards Association) as the standard specification for interfaces which can handle both analog and digital signals, is also steadily becoming popular. Within DVI there is DVI-I, which can accommodate both digital and analog signals, and DVI-D, which only accommodates digital TMDS signals, and care is taken to ensure that a DVI-I connector plug cannot be inserted into the digital DVI-D connector receptacle on the display apparatus side. However, despite the popularization of DVI, a large number of hosts carrying D-sub analog interface remains.
Because of this, display apparatuses are being produced where the display apparatus is equipped with a DVI-I receptacle able to accommodate both digital and analog signals, and if the host has a D-Sub connector, a D-Sub to DVI-I conversion cable is employed, and if the host has a DVI-D connector, it is connected employing a connection cable with both ends DVI-D.
In a display apparatus equipped with a DVI-I receptacle able to accommodate both analog and digital video signals, in order to realize a plug and play function it is necessary to have a total of two (nonvolatile memories), namely a first nonvolatile memory containing EDID for analog and a second nonvolatile memory containing EDID for digital. However, with the D-Sub connector of the analog interface, and even with a DVI-I connector, the DDC communication line that reads the EDID is only equipped with one line each for SCL and SDA, in other words, enough for only one channel. Therefore, in a display apparatus equipped with a DVI-I receptacle able to accommodate both analog and digital video signals, it is necessary to have a configuration that connects each of the nonvolatile memories for analog and for digital, and the SCL and SDA lines via a multiplexer, and selectively uses a DDC communication line of one channel by switching the multiplexer, to thereby transmits the EDID data to the host.
Moreover, it is necessary to operate the switching circuit made up of this plurality of nonvolatile memories and the multiplexer through a power source (DDC 5V) which the host side supplies to the display apparatus through a connection cable. This is in order to realize the plug and play function, even if the user starts the host first, before the display apparatus.
However there are cases in which, when the switching of the multiplexer is not appropriate, the EDID for digital is sent while connected to an analog interface, or conversely the EDID for analog is sent while connected to a digital interface. If the host receives this kind of incorrect EDID the appropriate driver software cannot be selected, and due to this the screen will not display an image at all, or alternatively does not give the correct display.
In order that the multiplexer can be switched by a voltage of DDC 5V only, then heretofore a method has been proposed where, as shown in FIG. 5, the user selects beforehand which EDID is to be read by means of a control key 61 of the display apparatus, and this selection result is stored in a third nonvolatile memory 52, and the multiplexer 21 is then switched based on this information. In the drawing, reference numeral 22 denotes a nonvolatile memory in which the analog EDID is stored, reference numeral 23 denotes a nonvolatile memory in which the digital EDID is stored and reference numeral 16 denotes a DVI-I receptacle 16.
Moreover, FIG. 6 is a drawing showing the configuration of a display system disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-175230. This method disconnects or grounds (disconnected in the figure) the DDC 5V of the D-Sub to DVI-I conversion cable 56 connected to the host 11 for analog signals, and automatically switches the multiplexer 21 based on the voltage of the DDC 5V. A rectifier circuit 54 for analog synchronization signals is added, and when the D-Sub to DVI-I conversion cable 56 is connected, power to the nonvolatile memory 22 which stores the analog EDID is turned on by the output from this rectifier circuit 54 as a power supply, to switch the multiplexer 21 to the analog EDID side. Reference numeral 10 denotes the host for the digital signal, reference numeral 14 denotes the DVI-D to DVI-D cable for connecting the DVI-I receptacle 16 and the host 10, and the DDC 5V is not disconnected or not grounded.
However, in the configuration of FIG. 5, the user is required to operate the settings using the control key 61 on the display. Moreover, if the settings are not correct, not only is the correct EDID not sent, but it is necessary to correct the third nonvolatile memory 52, requiring even further complex operations.
Moreover, in the method of FIG. 6, since the user does not perform the setting, the operation is not complex. However in a system in which the host side first reads the EDID and then outputs the video signal, the wrong EDID may be sent via the multiplexer when switched in a situation where an output from the rectifier circuit 54 has not been obtained. Furthermore in the method of FIG. 6, a rectifier circuit for the synchronized signal is necessary, and there is a problem in that the synchronized waveform becomes disordered or dulled due to the substrate wiring to the diode, or the capacitance between the terminals, thus affecting the image quality. Especially with flat panel displays, represented by liquid crystal displays, it is necessary to perform an AD conversion on the analog video signal, and the sampling clock for the A/D conversion is generated with the horizontal synchronization signal as a reference. Consequently there is a problem in that the disordering or dulling of the synchronized waveform amplifies the phase shift of the sampling clock, so that the change in the sampling point of the video signal becomes visible as the brightness changes, and so-called phase noise worsens.
In addition, according to the method of FIG. 6, since the DDC 5V of the D-Sub to DVI-I conversion cable 56 is disconnected, then when using this cable with a display apparatus that does not have the rectifier circuit 54, there is a problem in that the plug and play function cannot be realized, that is to say, the generality of the cable is low.